(1) Field of the Invention
The present invention relates to an image layout processing method for simulating on a CRT screen an arrangement of graphics, pictures, characters and the like (hereinafter referred to as objects) to be transferred to a prepress mechanical, and more particularly to a method of and apparatus for moving a key object as inserted between other objects displayed on the CRT screen.
(2) Description of the Related Art
The manner of moving a key object between other objects on a screen according to a conventional image layout processing method of this type will be described with reference to FIG. 1.
In FIG. 1, numeral 1 denotes an image memory plane on which a background object A is recorded (hereinafter referred to as a background memory plane). Numeral 2 denotes an image memory plane on which a key object B is recorded (hereinafter referred to as a key object memory plane). Numeral 3 denotes an image memory plane on which a foreground object C is recorded (hereinafter referred to as a foreground memory plane). Priorities are predetermined for the memory planes 1-3 based on overlapping conditions of the objects A-C. In this example, priorities are set in the descending order from memory plane 3 to memory plane 1.
The memory planes 1-3 are read in synchronized raster scans successively from respective leading addresses S1-S3. Pixel data read from the memory planes 1-3 are applied to a multiplexer 4 in a next stage. When the pixel data read from the memory planes 1-3 are all "ON", the multiplexer 4 selects and outputs the pixel data from the memory plane of the highest priority. If the pixel data from the memory plane of higher priority are "OFF", the multiplexer 4 outputs the pixel data from the memory plane of the next priority.
By fetching the pixel data from the memory planes 1-3 through the multiplexer 4 in this way, the objects A-C are presented overlapping one another on a display screen 5. As shown in FIG. 1, the leading address S2 for the raster scan of the key object memory plane 2 may be shifted to S2' while fixing the leading addresses S1 and S3 in the background memory plane 1 and foreground memory plane 3. This step allows the key object B to be moved between the background object A and foreground object C.
However, the conventional method described above has the following disadvantages.
The conventional method requires a plurality of image memory planes corresponding to the number of objects in order to display the objects overlapping one another on a CRT screen. Since these image memory planes must be raster-scanned independently of one another, an image processing apparatus implementing this method inevitably has a complicated construction. Complication and high cost of the apparatus may also be ascribed to the complexity of the multiplexer which selects the pixel data received from the respective memory planes according to the priorities.
Further, when an attempt is made in the prior method to display the key object B of FIG. 1 in front of the foreground object C, image data must be rewritten for exchange between the key object memory plane 2 and foreground memory plane 3, or the image data of the key object B must be written to an additional memory plane of higher priority than the foreground memory plane 3. This is an irksome operation in either case.